1. Field of the Invention
The present invention is related to an imaging apparatus equipped with a flash photography function, and a method for controlling flash. More specifically, the present invention is related to an imaging apparatus that obtains images with an imaging element which is capable of controlling an initiation timing and a cessation timing of charge accumulation in pixel units, and a method, and optimizes flash emission amounts utilizing the images, and a method for optimizing flash emission amounts.
2. Description of the Related Art
Recently, imaging elements which are provided in digital cameras and the like are transitioning form CCD image sensors to CMOS image sensors. CMOS image sensors can be manufactured by a process similar to that for manufacturing CMOS LSI's. Therefore, providing built in circuits other than image sensors into the same chip is facilitated, and CMOS image sensors are suited for use as so called “systems on chips”. In addition, CMOS image sensors have advantages over CCD image sensors in that there is less influence by noise (smearing) during signal transfer, and the number of power sources can be reduced.
On the other hand, CMOS image sensors cannot output signals from all pixels simultaneously due to their structures, unlike CCD image sensors. Therefore, signal readout is performed line by line or pixel by pixel. At this time, charge accumulation is initiated pixels from the point in time that signals are output therefrom. Therefore, the charge accumulation initiation timing and the charge accumulation completion timing of each pixel differ according to the signal output timing thereof.
Meanwhile, there is a known method for controlling flash of digital cameras, in which flash is emitted prior to actual photography (hereinafter, referred to as “preliminary emission”), and an image obtained during the preliminary emission is utilized to optimize flash emission during actual photography (hereinafter, referred to as “main emission”). This method is effective when the imaging element of the digital camera is a CCD, but not suited for use in the case that the imaging element is a CMOS image sensor, in which the charge accumulation initiation/completion timings are different among pixels. For this reason, digital cameras which are equipped with CMOS image sensors are further provided with light adjusting sensors, and flash is controlled based on data obtained by the light adjusting sensors.
Japanese Patent No. 3639734 discloses a method, in which flash emission is controlled by utilizing images obtained by a CMOS image sensor. This document proposes a method in which the timing of preliminary emission is adjusted at the factory which manufactures digital cameras, such that preliminary emission is performed during a period in which pixels within a specific block are accumulating charges. Then, the signals which are read out from the specific block are employed to control main emission.
The method disclosed in Japanese Patent No. 3639734 is that which reads out signals from the specific block at the preliminary emission timing, as described in paragraphs 0028 through 0030. In this method, it is necessary to control readout operations depending on whether preliminary emission is performed. However, there is a possibility that frequent switching of control methods will lead to problems such as operating errors, which is not preferable. In addition, preliminary emission and optimization of actual flash emission amounts are performed after a shutter release button is depressed, and therefore high speed processing is required. Therefore, complex processes that accompany frequent switching of control methods is not preferable from the viewpoint of processing time as well.